1. Field of the Invention
This invention relates to the press fabrics used in the press section of papermaking and similar machines to support, carry, and dewater the wet fibrous sheet as it is being processed into paper. The invention more specifically relates to open-ended press fabrics which are closed to assume an endless form by means of a pin seam during installation on the papermachine. It particularly relates to the use of unique yarns for the machine direction (MD) strands of the press fabric.
2. Description of the Prior Art
Endless fabrics are key components of the machines used to manufacture paper and similar products. In the present discussion, the fabrics used in the press section will be of primary concern. Not only do those fabrics function as a form of conveyor belt carrying the wet fibrous sheet being processed into paper through the press section, but, more importantly, they also accept water that is mechanically pressed from the sheet as they pass together through the presses.
At one time press fabrics were supplied only in endless form; that is, they were woven in the form of an endless, seamless loop. This was, in part, made necessary by the limitations of seaming and weaving technology. In addition, however, conditions in the press section present additional special requirements that would have to be satisfied in a workable seamed press fabric.
Historically, most of the methods for joining the ends of open papermachine fabrics, especially those used on the drying section of the machine, involve a seam which is much thicker than the rest of the body of the fabric. Such a seam would prove to be totally unworkable for a fabric used in the press section. A seam, thicker than the body of the fabric whose ends it joins would be subjected to elevated compressive forces on each passage through the press nip. This repetitive stress would weaken the seams and lead to reduced fabric life. Of potentially more serious consequence would be the vibrations set up on the press machinery by repetitive passages of the thicker seam region. Finally, the wet fibrous sheet, still quite fragile in the press section because of its high water content, can be marked, if not broken, where it comes into contact with a seam, because of these elevated forces of compression.
Despite these considerable obstacles, it remained highly desirable to develop an on-machine-seamed (OMS) press fabric, because of the comparative ease and safety with which it can be installed on the machine. This simply involves pulling one end of the open-ended press fabric through the machine, around the various guide and tension rolls and other components. Then, the two ends can be joined at a convenient location on the machine and the tension adjusted to make the fabric taut. In fact, a new fabric is usually installed at the same time as an old one is removed. In such a case, one end of the new fabric is connected to an end of the old fabric, which is used to pull the new fabric into its proper position on the machine.
By way of contrast, the installation of an endless fabric on a press section is a difficult and time-consuming undertaking. The machine must be shut down for a comparatively longer period while the old fabric is cut out or otherwise removed. The new fabric then must be slipped into proper position from the side into the gaps between the presses through the frame and-around other machine components. The difficulty of this procedure is further compounded by the fact that the newer press fabrics are gradually becoming thicker and stiffer. These characteristics add to the time and effort required on the part of plant personnel to install a new one. In this connection, a workable on-machine-seamable press fabric was an advance long sought by the industry.
Seamed press fabrics have now been in use for several years. One method to produce an open-ended fabric, that can be joined on the paper machine with a pin seam, is to weave the fabric in such a way that the ends of the machine direction (MD) strands can be turned back and woven into the body of the fabric and parallel to the machine direction. Such a fabric can be referred to as having been "flat" woven. This provides the loops needed to form the pin seam, so called because it is closed by means of a pin, or pintle, passed through the space defined by the alternating and intermeshing loops of machine-direction (MD) yarn at each end of the fabric when the ends are brought into close proximity to each other during closure.
Another technique employs the art of weaving "endless", which normally results in a continuous loop of fabric. However, when making a pin-seamable press fabric, one edge of the fabric is woven in such a way that the body yarns form loops, one set of alternating loops for each end of the woven cloth. In using either of these techniques, the seam region is only slightly thicker than the main body of the fabric, because the loops themselves are formed using machine direction (MD) yarns. This makes the pin seam a workable option for closing a fabric to be used on a press section.
Single monofilament strands have normally been used in both the machine and cross-machine directions of seamable press fabrics. The relative stiffness of monofilament ensures that it will have the requisite good loop formation properties. Experience has shown, however, that monofilament is difficult to weave and has insufficient elasticity in the machine direction for many kinds of contemporary presses. Tensile failure and seam breakage have been frequently observed.
Another difficulty is presented by the very open, rigid, incompressible structure of base fabrics woven from monofilament. For some papermaking applications, this incompressibility is not a problem, and may even be ideal. However, for positions that have poor auxiliary fabric dewatering capacity, or produce mark-sensitive sheets, a softer, more compressible base fabric is needed.
Historically, a more compressive base fabric would have been achieved by weaving with multifilament yarn, rather than monofilament. Yet, these yarns do not have the rigidity necessary for good loop formation or to maintain the integrity of the seam area during loop meshing when closing the seam upon installing the fabric on a papermachine.
The present invention is designed to overcome this shortcoming of multifilament yarn by providing a yarn which has the characteristics needed for good loop formation and meshing during seaming as well as compressibility and elasticity in the machine direction.